Some coal-fired boilers are provided with a regenerative rotary preheater which preheats, through heat exchange with flue gas from the boiler, primary and secondary airs (primary and secondary combustion support fluids) for supply of pulverized coal to the boiler and for stable combustion of the boiler, respectively, and supplies the primary and secondary combustion support fluids to the boiler.
Moreover, an oxyfuel combustion boiler has been proposed nowadays which uses not air but oxygen to burn the pulverized coal. In the oxyfuel combustion boiler, flue gas mainly composed of CO2 (carbon dioxide) is drawn from the oxyfuel combustion boiler as primary and secondary recirculation gases (primary and secondary combustion support fluids) which are guided to a regenerative rotary preheater and are preheated through heat exchange with the flue gas. The secondary combustion support fluid upstream of the regenerative rotary preheater is mixed with oxygen from an air separation unit (ASU) and the secondary combustion support fluid mixed with the oxygen is supplied to the boiler. In oxyfuel combustion of pulverized coal in the oxyfuel combustion boiler, a flow rate of the secondary combustion support fluid 12 to be mixed with the oxygen may be controlled to properly control a combustion temperature of the oxyfuel combustion boiler.
The oxyfuel combustion boiler has attracted attention as an effective device for withdrawal and disposal of CO2 since flue gas mainly composed of CO2 (carbon dioxide) is discharged from the oxyfuel combustion boiler in the oxyfuel combustion of the pulverized coal as mentioned in the above.
The flue gas of the oxyfuel combustion boiler, which is induced by an induced fan, has negative pressure (low pressure). By contrast, the secondary combustion support fluid, which is boosted in pressure by a forced draft fan for supply to the oxyfuel combustion boiler, has a given pressure (medium pressure); the primary combustion support fluid, which is passed through the mill to transport the pulverized coal to the oxyfuel combustion boiler, is boosted in pressure by boost-up and primary draft fans into a highest pressure (high pressure). Specifically, pressure relationship is: pressure of the flue gas<pressure of the secondary combustion support fluid<pressure of the primary combustion support fluid.
The regenerative rotary preheater is provided with sector plates arranged adjacent to axially opposite ends of a rotating rotor, respectively, to compart openings at the axially opposite ends of the rotor. The sector plates compart a flue gas flow passage through which passed axially of the rotor is the low-pressure (negative-pressure) flue gas, a primary-combustion-support-fluid flow passage through which passed axially of the rotor is the high-pressure primary combustion support fluid comprising the recirculated flue gas boosted to the given pressure and a secondary-combustion-support-fluid flow passage through which passed axially of the rotor is the medium-pressure secondary combustion support fluid comprising the recirculated flue gas mixed with the oxygen and having the pressure higher than that of the flue gas and lower than that of the primary combustion support fluid, adjacent to one another. The rotor filled with a great number of thermal storage plates has partition plates radially extending to circumferentially partition the rotor into many sections. Each of the partition plates has opposite ends axially of the rotor each having a seal plate for gas sealing with the sector plate.
The flue gas is passed through the flue gas flow passage in the regenerative rotary preheater from above to below whereas the primary and secondary combustion support fluids are passed through the primary- and secondary-combustion-support-fluid flow passages from below to above, thereby providing the mutually opposed flows. The flue gas flow passage and the primary- and secondary-combustion-support-fluid flow passages are mutually partitioned in a gas sealed manner by the sector plates and the seal plates.
However, between the flue gas and the primary and secondary combustion support fluids flowing through the regenerative rotary preheater, there is direct leak of gas in relatively large quantity from higher to lower pressure sides through between the sector plates and the seal plates. There is also entrained leak of gas to an adjacent flow passage in an entrained manner in association with rotation of the rotor. Leaked quantity by the entrained leak is small in comparison with that by the direct leak.
The medium-pressure secondary combustion support fluid in the secondary-combustion-support-fluid flow passage provides direct leak in relatively large quantity to the neighboring flue gas flow passage through which passed is the low-pressure flue gas. As a result, when the secondary combustion support fluid preliminarily mixed with the oxygen leaks to the flue gas flow passage, the leaking oxygen is uselessly discharged together with the flue gas with no contribution to fuel combustion. Thus, the more the oxygen leaks to the flue gas, the more a capacity of the air separation unit (ASU) must be increased, leading to a problem of increase in installation and running costs.
In order to prevent the secondary combustion support fluid in the secondary-combustion-support-fluid flow passage from directly leaking to the flue gas flow passage, it has been proposed to position two primary-combustion-support-fluid flow passages between a flue gas flow passage and a secondary-combustion-support-fluid flow passage (Patent Literature 1).